1. Field of the Invention
The present invention relates to a technique capable of reducing an amount of noise included in an image. More particularly, the present invention relates to a technique capable of reducing noise by adding a time-directional image component.
2. Description of the Related Art
In the field of X-ray moving image capturing technology, it is conventionally known that a two-dimensional X-ray moving image capturing apparatus can convert incident X-ray into visible light with a combination of a fluorescent substance and an image intensifier. The image capturing apparatus further includes a TV camera equipped with a charge coupled device (CCD) sensor to capture a visible light image. Another X-ray moving image capturing apparatus that includes a flat panel detector (FPD) operable as an image capturing system is also conventionally known.
However, a moving image obtained by the above-described X-ray moving image capturing apparatus (i.e., a set of a plurality of continuously collected still images, which are hereinafter referred to as “frames”) is not free from quantum noises that may be generated by X-ray quantum fluctuations or system noises that may be generated by a detector or a circuit. These noises are superposed on the moving image in sequential image capturing processes.
In particular, the influence of the above-described noises is not negligible when the X-ray moving image capturing apparatus is used in the medical field, because an image capturing operation is performed with a lesser amount of X-ray from the point of view of preventing a measuring object person from being exposed to the X-ray. In this respect, it is important to reduce the noises to improve the image quality.
Therefore, in the field of image processing, it is conventionally proposed to perform weighting addition processing (so-called filtering processing) on pixel values in the time direction or in the space direction to reduce the above-described noises.
More specifically, the time-directional weighting addition processing can reduce the fluctuation of temporally uncorrelated noises by averaging a pixel value of a target frame and a corresponding pixel value of a past frame. Further, the space-directional weighting addition processing can reduce the fluctuation of spatially uncorrelated noises by averaging pixel values of a target pixel and neighboring pixels.
The time-directional weighting addition processing is an ideal method if there is not any temporal signal change between frames, which may be caused by the movement of a measuring object person. However, if a signal change occurs, the image of a past frame (i.e., an afterimage) will appear on the target frame.
In this respect, the space-directional weighting addition processing does not refer to the past frame. Therefore, no afterimage appears. However, the space-directional weighting addition processing is not a desired method in that a spatially non-similar structure (such as an edge boundary) becomes unclear together with the noises.
In view of the foregoing, a method for adequately combining the time-directional weighting addition processing and the space-directional weighting addition processing is discussed in Japanese Patent Application Laid-Open No. 6-47036. The method discussed in Japanese Patent Application Laid-Open No. 6-47036 includes detecting a movement based on a local average difference between a target frame and a past frame and switching the weighting addition processing from the time direction to the space direction if the detected movement is large. The method discussed in Japanese Patent Application Laid-Open No. 6-47036 can realize an ideal noise reduction if there is not any movement, and can suppress the generation of an afterimage in the noise reduction even if there is a movement.
However, when the weighting addition processing is switched between the time direction and the space direction as described above, it is important to accurately extract a movement from a noise containing signal. However, according to the method discussed in Japanese Patent Application Laid-Open No. 6-47036, the influence of a contained noise component is not sufficiently considered.
More specifically, according to the method discussed in Japanese Patent Application Laid-Open No. 6-47036, a movement is detected based on the local average difference between the target frame and the past frame. However, the method cannot strictly determine whether the difference is derived from a temporal signal change caused by the movement of a measuring object person or from a signal change caused by temporal noise fluctuations. Thus, the method discussed in Japanese Patent Application Laid-Open No. 6-47036 cannot accurately determine the movement.